1. Field of the Invention
The present invention relates to a sheet processing apparatus which processes a sheet and an image forming apparatus provided with the sheet processing apparatus.
2. Description of the Related Art
Conventionally, in an image forming apparatus such as a copying machine, a printer, and a facsimile, there is disclosed a sheet processing apparatus wherein sheets to which images have been already formed are sequentially taken in the apparatus and a binding process is performed to the sheets (for example, see Japanese Patent Application Laid-Open No. 2004-59314).
For example, in a conventional sheet processing apparatus shown in FIG. 16, an intermediate roller discharges and stacks the sheet to and on an intermediate stacking portion. An alignment member presses the stacked plural sheets from an opposite side against an alignment reference wall fixed onto a stapler side, and thereby the sheets are aligned by causing the sheets to abut on the alignment reference wall. The stapler performs the binding process to the aligned sheets.
For the alignment in a sheet conveyance direction, the sheets are aligned by a mechanism such as a paddle and a roller which abuts on a sheet upper surface to move the sheet in an opposite direction to the conveyance direction.
In the case where the sheets having different lengths in a sheet width direction are aligned like the sheet processing apparatus of FIG. 16, a pressing amount (alignment amount) of the sheet by the alignment member is set according to a sheet size.
In the apparatus which can align an A4-size sheet and a letter-size sheet conveyed based on the center in the sheet width direction (center reference), when the sheet is longitudinally conveyed, the width of the letter size is about 216 mm, and the width of the A4 size is 210 mm. In the alignment amount of the alignment member, the A4 size is larger than the letter size by about 3 mm in a range of a sheet end portion to the alignment reference wall because the letter size is wider than the A4 size by about 6 mm.
When the sheet is transversely conveyed, the width of the letter size is about 279 mm and the width of the A4 size is 297 mm. Therefore, in the alignment amount of the alignment member, the letter size is larger than the A4 size by about 9 mm in the range of the sheet end portion to the alignment reference wall.
However, in the case where the sheet having the wrong size is conveyed due to a setting mistake at the sheet cassette, the alignment member aligns the sheets with a correct amount, and misalignment is possibly generated. A sheet regulating member for regulating an end of the sheet is provided in the sheet cassette, and a size of the sheet in the sheet cassette is distinguished by detecting a position of the sheet regulating member. The sheet regulating member is displaced by a user, even if the sheet having different size is set in the sheet cassette, the sheet regulating member may not be displaced. And in case the sheet regulating member is set in the wrong position, a size of the sheet in the sheet cassette is distinguished into different size.
Particularly, in the case where the letter-size sheet and the A4-size sheet are transversely conveyed, even if a sheet regulating member is set at the A4-size position in the sheet cassette, the letter-size sheet differs slightly from the A4-size sheet in a length of about 6 mm in the sheet conveyance direction. Therefore, the letter-size sheet can actually be accommodated. However, the letter-size sheet differs from the A4-size sheet by about 18 mm in the sheet width direction. Such the large difference in alignment amount has a large influence, as discussed below.
When the sheet is conveyed to the intermediate stacking portion, the alignment member stands by at the position having a predetermined distance away from one of the end portions of the sheet. In this case, it is assumed that the distance is 7 mm.
The stapler is placed outside a conveyance area so as not to obstruct the conveyance of the sheet, and the alignment reference wall is placed with respect to the stapler such that the sheet is aligned with a predetermined position. Therefore, the alignment reference wall is placed at the position separated away from the other end portion of the sheet by a predetermined distance. It is assumed that the distance from the end portion of the A4-size sheet is 20 mm. In the necessary alignment amount with which the alignment member causes the sheet to abut on the alignment reference wall, a distance is 7 mm from the standby position to the position where the alignment member abuts on one of the end portions of the sheet, and the a distance is 20 mm until the other end portion abuts on the alignment reference wall since the alignment member presses the sheet. Accordingly, the necessary alignment amount becomes 7+20=27 mm for the A4-size sheet, and the necessary alignment amount becomes 7+20+9 (difference in width direction between the A4-size sheet and the letter-size sheet)=36 mm for the letter size sheet.
In the case where the letter-size sheet is conveyed while wrongly detected as the A4-size sheet, when the sheet is conveyed to the intermediate stacking portion, the alignment member is located at the position which is 9 mm away from the original standby position where the alignment member is located in conveying the letter-size sheet. The distance of 9 mm corresponds to the difference in sheet width direction between the A4-size sheet and the letter-size sheet. Furthermore, because the alignment amount is decreased by about 9 mm, the sheet is moved only to the position which is about 18 mm away from the alignment reference wall.
The stapler cannot staple the sheets located at the position which is 18 mm away from the alignment reference wall, and the staple of the stapler strikes air. Accordingly, not only are the sheets not stapled, but also the staple which was intended for the sheets remains in the apparatus. When the phenomenon is frequently generated, misalignment, a jam, and breakage of the stapler are caused by the large amount of staples remaining in the apparatus.